The present invention relates to a method and a system for determining the location of a mobile subscriber registered in a cellular mobile radiotelephone network for an optimal routing, given an incoming call directed to the mobile subscriber.
A cellular radiotelephone network, such as for example the digital radio communication system defined according to the GSM standard (Global System for Mobile Communication), is constructed from a plurality of radio cells, and offers the mobile subscriber a maximum degree of mobility in the context of radio communication. The international mobile radiotelephone network according to the GSM standard comprises a base station system for setting up, dismantling, and maintaining radio links between mobile subscriber stations and base stations, a switching-oriented system for setting up, managing and clearing down connections, taking into account mobility-specific functions, as well as an operating and maintenance system for monitoring operational states and for controlling the network components arranged in distributed fashion in the mobile radiotelephone network. The tasks of the switching-oriented system include the management of the subscriber data of the mobile subscribers, the discovery of the location of a called mobile subscriber or, respectively, of his mobile subscriber station at any location of the GSM service area, the connection setup and the acquisition of the fee data (charging of a call) for calls going out from or, respectively, coming in to the mobile subscriber. To carry out the tasks, the switching-oriented system is provided with mobile switching centers which in particular take over the mobility-related methods and functions in the setting up and dismantling of connections and in handling connections within the mobile radiotelephone network and at the interfaces to other communication networks, as well as via visitor registers and home registers, of which the visitor register temporarily contains the subscriber data of the mobile subscribers already located in the service area of an allocated mobile switching center, while in the home register the subscriber data of the mobile subscribers are permanently managed.
The subscriber data determine the subscriber profile, that is, the services and authorizations booked by the mobile subscriber. In addition, the home register stores an item of information for the identification of the visitor register in which the mobile subscriber is managed at that moment on the basis of his location. As a rule, this is the address of the visitor register or, respectively, of the mobile switching center, so that the location of the mobile subscriber can be determined by a location query directed to the home register by the mobile switching center. Via the address of the currently responsible visitor register, the home register can request a mobile subscriber roaming number, which is required to enable the setup of a connection from a gateway mobile switching center to a visited mobile switching center.
To support an optimal routing in the GSM mobile radiotelephone network, according to the GSM recommendation 03.79, version 0.9.0 of Jul. 31, 1996, a location query message can be sent across networks by the gateway mobile switching center of a first network to the home register of another network in which the called mobile subscriber is permanently registered with his subscriber data. The purpose of the location query is to provide information on the basis of which a direct connection can be set up to a visited mobile switching center of another network in which the called mobile subscriber is currently registered on the basis of his location.
According to the cited GSM recommendation 03.79, in order to support the optimal routing a two-stage location query is required, in which at first only items of information concerning the location of the called mobile subscriber are requested, e.g. in the form of the address of the responsible visited mobile switching center, by the gateway mobile switching center with a first query (send location info) directed to the home register of the other network. For this purpose, a message concerning the request of items of subscriber information (provide subscriber info) is sent by the home register to the visitor register currently responsible for the mobile subscriber, and as an acknowledgment a message (provide subscriber info acknowledge) is received. The home register thereupon sends an item of information for the identification of the visited mobile switching center back to the gateway mobile switching center in a message (send location info acknowledge) with which the first request is acknowledged.
After this, the gateway mobile switching center checks to see that nothing stands in the way of the setting up of a direct connection to the visited mobile switching center for reasons of charging. The requirements for the charging of the call with support of the optimal routing are described in the GSM recommendation 03.79, on page 24 in chapter 9.1. If the result of the check is positive, a second location query (send routing info) is directed to the home register by the gateway mobile switching center, so that the visitor register can request the mobile subscriber roaming number. The advantage of the two-stage location query, which respectively has to be initiated by the gateway mobile switching center, is that the mobile subscriber roaming number is first requested by the visitor register when, after checking the charging requirements, the use of a mobile subscriber roaming number for the connection setup is ensured, and thus the assignment of mobile subscriber roaming numbers that remain unused due to unfulfilled charging requests is avoided.